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Euphorbia—Part 7
In this final installment on the genus Euphorbia, I’m going to concentrate on the unique euphorbia population native to the enormous island of Madagascar. Madagascar is famous for its population of endemic mammals, lemurs in particular, and unfortunately is notorious as one of the world’s prime examples of environmental destruction and habitat loss. Less widely known, however, its vegetation is as distinct as its fauna, and a major component of that vegetation is a large population of succulent plants. One of these, the family Didiereaceae, grows nowhere but Madagascar, and while the same certainly can’t be said for its euphorbias, most Madagascan euphorbias are so distinctive that anyone even somewhat acquainted with succulent plants can immediately recognize a euphorbia from the giant island.
The driest part of Madagascar is its southwest corner, and the extensive arid regions of the island are covered with tree sized pachycauls from several genera of Didiereaceae, interspersed with pachypodiums and succulent, tree-sized euphorbias. These euphorbias are similar to the very first ones discussed in this series, with succulent stems and branches, but not that highly modified morphologically in terms of body segments, tubercles, spines and so forth. In one well-known species, Euphorbia stenoclada, the branch tips are hardened into sharp points, as is the case with some of the less developed South African species, such as E. lignosa, but this form of spination has nothing in common with either the peduncular or stipular spines found in the true spiny euphorbias. These Madagascan euphorbias may have a common succulent ancestor with similar South African species, or they may have evolved separately from non-succulent euphorbias as a response to climate and environmental change.
The spiny euphorbias of Madagascar form a very distinct group, quite unlike species from anywhere else. Though their spines are stipular, they may have evolved this defensive mechanism quite independently from those species from mainland Africa, and within the genus they have been given their own sub-generic designation.
By far the best known of the Madagascar euphorbias, spiny or not, is Euphorbia milii, grown throughout the world, and often called the “crown of thorns.” The typical form is a spiny bush, densely branched, with pairs of spines, woody, not particularly succulent stems covered with thin green leaves, and flowers distributed near its branch tips, typical non-descript euphorbia blossoms surrounded by a pair of bright red bracts. E. milii exemplifies the aspect taken by many Madagascan euphorbias, and the species itself has a number of forms and varieties, such as var. breoni, with heavily spined woody stems twice as thick, and with larger bracts produced in larger numbers. E. milii var. imperatae, on the other hand, is a miniature form that, though ultimately capable of growing to about a foot and a half tall and wide, will remain content and flowering in a three inch pot for decades—in every other way than size it resembles a standard E. milii. Other varieties of E. milii, less frequently encountered in cultivation, may have flowers with yellow or even white bracts. As a popular horticultural plant, the crown of thorns has probably been over described, with many minor forms, bigger plants, more free flowering, or more compact growing, and so forth, being given some kind of official (though perhaps dubious) taxonomic recognition. Over the last several years a number of hybrids involving E. milii have come onto the market, many developed in Thailand. These may be larger stemmed plants, generally with much larger flower bracts and flowers in various pastel shades of pink, orange and yellow.
Other plants, very similar and differing primarily in bract color, arrangement and number, or size of the bracts, have also been described, including Euphorbia durani and the very slow-growing E. horombensis. When mature, these species and their near-relatives tend to form hemispherical mounds up to a yard across, but their ultimate growth form isn’t likely to come about when confined to a container. Slightly farther afield from the E. millii model, we get species such as E. croizatii and E. delphinensis, with rounder leaves, and less gaudy bracts. As more parts of the island are carefully explored, local and small-growing species of euphorbia keep being discovered. These include rarities such as E. guillemetii, with small, pinkish bracts, and the fairly recently discovered E. hofstaetteri, with small, variably colored bracts, a tuberous root, thick, spiny stems, and, in some forms, hairy leaves. Several species of these spiny euphorbias produce narrowly linear leaves, among them, E. rossii, with beige bracts, and the pale green-flowered E. genoudiana. Another linear-leafed species, recently discovered and still somewhat rare in cultivation, E. gottlebei, differs from the others in its large, bright red bracts, very similar to E. milii itself.
The list of these spiny plants goes on (I’m leaving out at least as many as I’m including), but a few of them really do seem quite different. Euphorbia didiereoides grows much like an ocotillo, with a fountain-like spray of ascending, arching branches up to seven or eight feet tall. Along with its main basal branches, it produces very distinctive short secondary stems. The bracts of its flowers enclose the flowers themselves like a tube. Also with somewhat enclosed flowers, but borne on long, sometimes pendent inflorescences, both E. perieri and E. pauliani do better with more water and a little less light than most of the others in this group. The related E. pedilanthoides, with flowers almost totally enclosed by swollen-based, tightly tubular, brightly colored bracts, forms a compact multi-stemmed plant with a distinctly thickened base, almost like a miniature pachycaul tree. The linear-leafed E. kondoi, another recent discovery, is a delicate grower which produces a large tuberous root.
Perhaps the most interesting of all these spiny, branching species is Euphorbia guillauminiana, confined to a few arid lava beds. This species regularly rebranches, with the new branches all of an equal length, ultimately making a nearly perfect hemispherical mound up to two feet in diameter and height. Cultivated plants slowly attain this form, but collected plants already have it, and command high prices, making one question the viability of the plant’s survival in the wild—on the other hand, habitat loss is so horrendous in Madagascar that leaving the plants alone may not be much help either.
Most of the plants that I’m lumping together in a group with Euphorbia milii grow very easily in cultivation. With a few exceptions, they like very bright light, and most do best with a somewhat richer soil than most succulents, water once a week (or even a bit more frequently) in warm weather, and water about every two weeks in winter. E. milii itself is marginally outdoor hardy in our area, but most of the others are both slow growing and somewhat rare, not good subjects to put at risk for experiments in frost-hardiness. A few of these species are more difficult. E didiereoides should be kept a bit drier during the winter, as should E. pedilanthoides and similar tuberous rooted species such as E. kondoi. In my experience E. rossii will suffer and possibly rot if watered too frequently in winter, and E. guillauminiana in particular requires very dry treatment after it drops its leaves; leaving it completely dry for a few months until it makes its new growth won’t hurt it, although water every six weeks or so is probably all right also. These water-sensitive species will also benefit from a leaner, quicker draining soil mix.
Another group of related Madagascan euphorbias grows in higher rainfall regions than most succulents. Their flowers are small, tightly enclosed by tubular, overlapping, often vividly colored bracts. These plants may slowly branch or may remain solitary, growing into small columns, and their stems tend to spiral as they grow. They all produce large, rather tropical looking, though deciduous, foliage. Some, such as the various varieties of Euphorbia viguieri are fiercely armed with thick-based, sometimes jagged, often strikingly white spines that contrast with their bright green stems. Euphorbia pachypodioides, with a spiny, purple-green cylindrical body topped with a rosette of bright green leaves that are purple-red underneath, and with an erect inflorescence of a few dozen tightly arrayed little flowers that look almost like some sort of vegetable radar tower, was almost unobtainable for years until a famous grower wandered out to his greenhouse while drunk one night and somehow managed to successfully pollinate some plants. The species still isn’t common in cultivation.
Others in the group have spines reduced to fringes running down their stems, and one of these, Euphorbia leuconeura, is both self-fertile and extremely prolific. Its flowers lack attractive bracts, but when young it leaves are marked with bright white veins. This plant grows in areas of high rainfall, and I’ve seen a habitat picture of it growing epiphytically in the east Madagascan rainforest. It’s sometimes sold as E. lophogona, a much less common (although certainly not rare) species that is not self-fertile. Its fringe “spines” are more fibrous than in E. leuconeura, its stems are brown and woody looking, its leaves are dark green and leathery, and its flowers are surrounded by white or occasionally pink bracts. E. neohumberti is a species with a stocky green trunk, decorated with spiraling patterns of white leaf scars and bristly spines much like E. lophogona. Its enclosing floral bracts are bright red; in E. aureoviridiflora, the bracts are yellow-green, though otherwise young plants look almost identical. As it matures, the latter species sends out lateral branches and forms a hemisphere, while E. neohumbertii usually remains unbranched. Most cultivated plants of this group are small, but given time and root-room they will often get to three feet or more in height, with proportionately stocky stems. Out of leaf, from a distance, they seem more cactus-like than most Madagascan euphorbias, but they take much the same care as the E.milii group; a fairly rich soil, bright light, and water once a week while in leaf and every two week or so after the leaves drop in late fall or winter.
A few small species of Madagascan euphorbias resemble some of the South African plants which remain mostly underground, producing annual rosettes of leaves. The flowers of the island species, however, show their relationship with the other Madagascan euphorbias, having small flowers enclosed or surrounded by decorative bracts. The best known of these geophytic (underground stemmed) species is Euphorbia primulifolia, with bright white bracts surrounding its flowers, which begin to bloom a little before its new leaves appear. In the wild this species and rather similar relatives, such as E. quartziticola and the very rare E. subapoda, would look like nothing more than a little rosette of leaves accompanied by some small but rather pretty flowers. If cultivated, however, their tuberous bodies are generally raised above soil level, both for show and to keep them from rotting, and the little rosettes suddenly are transformed into fascinating caudiciforms. There are a few other described species with this habit, and as the plants are almost invisible much of the time (completely so when out of leaf), there may well be others yet undiscovered. As with most plants of this nature, fast draining soil and a bit of are when watering is best, but it’s perfectly possible to keep these little euphorbias happy for many decades.
Still another group of these island euphorbias consists of plants that have lost their spines entirely and exist as little smooth-stemmed, generally solitary columns. Many of these flower before they leaf out, and their inflorescences generally cover the top of the stem like a decorative hat. Among them, the best known may be Euphorbia ankarensis; others include E.milloti, with thinner, sometime branching stems and veined leaves, E. moratii and E. albertensis. They all are quite rare in cultivation, and need a little less light than most of the species mentioned so far. They enjoy a good deal of water when growing, but should be kept quite dry after their leaves fall.
A final group consists of typically low-growing species with somewhat succulent leaves. These plants crawl along the sandy dirt and undergrowth of their home, sometimes, as is the case with Euphorbia cylindrifolia, sending out underground shoots and rhizomes. A plant with small, terete, almost black leaves and apricot-colored bracts surrounding its flowers, E. cylindrifolia forms a mass of underground stems when confined to a pot, even sending new branches out through the drain holes at the bottom of a pot. Its’ much less robust subspecies tuberifera forms a caudex rather than rhizomes, as it grows painfully slowly, with slightly shorter leaves than its cousin, but identical flowers.
Several in this group have very decorative leaves, in many cases highly crisped and zigzagged along their margins, as in the low growing, freely branching Euphorbia decaryi, the slightly more erect E. capsaintemariensis, the tiny, extremely slow-growing E. tulearensis, and two rare, caudex-formers, E. ambovombensis and the excessively scarce, recently discovered E. suzannae-marnerianae. The leaves of E. francoisii, on the other hand are less ornately margined, but come in shapes ranging from almost like a diamond to nearly linear, in colors from plain green, to green lines with pink, to pink and silver, to almost pure silver with just a few pink and green veins, to randomly splotched with silver, pink and brown. The explosion of new species of this and some of the other Madagascan euphorbias results from a combination of extreme endemism in most of the species and a very low profile. The days of new discoveries always leads to a plethora of new species names, and given time, some of these new species may be sunk and combined with others. In any case, there is a world of unusual forms, shapes and colors in these many miniatures from the giant island.
Most of the succulent species do best with a little less light than the others, average (once a week) water in the growing season, and a little less water (perhaps every three weeks or so) in winter. They like a quick draining soil, but one with some decent nutrition and organic matter in it. On the whole, these euphorbias grow pretty easily, and many will never outgrow a three inch pot.
Most euphorbias from Madagascar are reasonably easy to grow—I’ve noted some of the exceptions. Most will grow from stem cuttings, although this is problematic and perhaps impossible in the case of the geophytes and though the species with tuberous bases will root from cuttings, it’s not clear if these cuttings will ever produce tubers (given many years, I expect most would). Since they don’t need too much light and many remain small, some of these plants make ideal window-sill growers. The bigger ones, though needing protection from cold, make a distinct addition to any succulent collection.
Since I’ve mentioned rooting cuttings and the problems with producing new tubers, I should add a bit about some of the South African plants before I conclude with the genus. Both the caudiciform twin-spined types, such as Euphorbia tortirama, and the medusa types, will root from stem cuttings, but generally the cuttings will just keep elongating rather than develop into a typical plant. The classic way to circumvent this is provided by these endless elongating cuttings themselves, as they often (particularly the medusa types) send out little multi-stemmed heads, and these can be detached, dried off, and rooted, and will grow into normal adult plants. It’s possible, though not as reliable, to do this with the caudiciforms as well (with a bit of luck), and E. knuthii, which is a sort of caudiciform, will make perfectly normal plants from a stem cutting without any additional effort. As always, when cutting euphorbias, be careful about the sap, wash your hands religiously after working with them, and avoid brushing your eyes, nose or mouth. Euphorbias differ wildly in terms of toxicity, and different people have differing sensitivities as well, but caution can’t hurt, and carelessness possibly can. Enough preaching.
The Garden has a fairly good collection of Madagascan euphorbias, including a few quite unusual species. We have some for sale (some really rare ones at the last sale) from time to time in limited numbers, so keep your eyes open for them.
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